An abnormal state of an X-ray computed tomography apparatus is detected. The abnormal state of the X-ray computed tomography apparatus can be confirmed by checking an error log or observing a reconstruction image. In this case, a change in the state of the apparatus cannot be noticed until the apparatus becomes abnormal. An abnormal state can be detected by performing a CT scan of a phantom for image quality analysis at the time of a routine check of the apparatus. To be more specific, a service technician checks an output of the X-ray detector or a reconstruction image, or automatic analysis is performed using analysis software. In this case, the state of the apparatus cannot be confirmed until a regular check is actually performed.